A continuing need in the pressure-sensitive adhesive (PSA) art is the achievement of better control over various mechanical and process properties so that adhesives can be "tailor-made" for specific, highly demanding end-use applications such as packaging, medical, and masking tapes. These applications require a proper balance of properties, and this balance changes with the particular end-use.
Among the earliest polymers to provide a reasonable balance of the properties required for satisfactory PSA performance were the natural and synthetic rubbers. However, these PSAs had poor aging properties, and the art quickly recognized the need to eliminate ethylenically unsaturated groups from the polymer backbone of rubber adhesives in order to improve their oxidative stability.
With the discovery of Ziegler-Natta (ZN) catalysts, it became possible to polymerize .alpha.-olefin monomers to high molecular weight. The homopolymers of the C.sub.6 -C.sub.10 .alpha.-olefins were naturally tacky and therefore good candidates for PSAs since they also had low toxicity, good aging and favorable environmental stability (i.e., low oxidation). These homopolymers were chemically inert, resistant to plasticizer migration, and relatively inexpensive. They, however, had poor cohesive strength and therefore, lacked the shear adhesion necessary for high performance PSAs.
Use of ZN catalysts to make homopolymers from .alpha.-olefin monomers, and to make copolymers from mixtures of .alpha.-olefin and nonconjugated polyene monomers is known in the art and is succinctly summarized in Odian, G., "Principles of Polymerization", Ch. 8.4 (Second Edition J. Wiley & Sons, New York, 1981). For a more detailed discussion of the polymerization of .alpha.-olefins, see Boor, J., "Ziegler-Natta Catalysts and Polymerizations", Ch. 19 (Academic Press, New York, 1979).
Adhesives derived primarily from C.sub.6 or higher .alpha.-olefins are also known. U.S. Pat. No. 3,542,717 describes poly(.alpha.-olefin) adhesive compositions comprising mixtures of polyolefin copolymers derived from olefin monomers with different molecular weights (i.e., copolymers from an .alpha.-olefin monomer having 11-20 carbon atoms and a different .alpha.-olefin monomer having 4-20 carbon atoms).
U.S. Pat. No. 3,635,755 describes polyolefin PSAs suitable for use as a single component PSAs for surgical tapes that are substantially non-allergenic. Such adhesives can be made from homopolymers of the C.sub.6 to C.sub.11 .alpha.-olefins or from inter-polymers of C.sub.2 to C.sub.16 .alpha.-olefins having an intrinsic viscosity of 1.5 to 7.0, a Williams plasticity of 0.8 to 4.0, and an acetone/heptane soluble fraction of less than 25% by weight. This patent does not teach that its polyolefin compounds are radiation curable.
U.S. Pat. Nos. 3,954,697 and 4,072,812 describe hot melt adhesives based respectively on propylene/C.sub.6 -C.sub.10 1-olefins wherein the C.sub.6 -C.sub.10 1-olefins comprise 40 to 60 mole percent of the composition and 1-butene/C.sub.6 -C.sub.10 1-olefins wherein the C.sub.6 -C.sub.10 1-olefins comprise 40 to 60 mole percent of the composition. These compositions have no detectable crystallinity by DSC (differential scanning calobrimetry). These two patents also teach that copolymers containing only minor amounts of propylene or butylene (e.g. 5-20 wt %) are permanently tacky. Moreover, they teach that copolymers containing only minor amounts of propylene or butylene have little static shear and fail in the static shear test after less than 100 minutes. In addition, neither patent teaches the use of polyene monomers.
U.S. Pat. No. 4,178,272 describes hot melt adhesives based on a blend of poly (propylene-co-higher 1-olefin) containing 35 to 65 mole percent higher 1-olefin. In addition, tackifying resin and crystalline polypropylene are added. Without the crystalline polypropylene homopolymer, the adhesive exhibits excessive creep under load.
U.S. Pat. No. 4,259,470 describes hot melt adhesives containing propylene, 1-butene or 1-pentene and 3 to 14 mole percent of at least one C.sub.6 -C.sub.10 linear .alpha.-olefin. Tackifying resins and plasticizing oils are also added to the hot-melt compositions.
U.S. Pat. No. 4,288,358 describes hot-melt adhesive compositions containing terpolymers of 10 to 55 mole percent propylene, 15 to 60 mole percent 1-butene or 1-pentene and 5 to 39 mole percent higher 1-olefins, i.e., C.sub.6 -C.sub.10 .alpha.-olefins. Tackifying resins and plasticizing oils are also added to the hot-melt compositions.
U.S. Pat. No. 5,112,882 describes a radiation curable poly (.alpha.-olefin)-containing adhesive composition that is pressure-sensitive at room temperature which upon cure yields an adhesive film having superior balance of peel and shear adhesion. The adhesive composition comprises (a) C.sub.6 to C.sub.10 .alpha.-olefin monomer and 0 to 15 mole percent of polyene monomers and (b) sufficient photoactive cross-linking agent to cross-link the composition upon irradiation. Sufficient radiation energy to generate free radicals is required to cross-link the composition.